Nowadays there exists an increasing interest for supports that have optical and/or energetic properties that can be controlled at will. It is particularly advantageous to be able to control the intensity of day light or the vision degree through a glazing.
In this context, some firms market glazing constituted of electrochromic materials that have the property of going from an oxidation state having a high light transmission to an oxidation state having a low light transmission.
Another widespread method in the field of so-called intelligent glazing consists in the preparation of PDLC films (polymer dispersed liquid crystal). PDLC films are composites films formed by liquid crystal droplets dispersed in a polymer matrix. Each droplet has its own orientation resulting in a global isotropic orientation and physically resulting in the PDLC film opacity. Under application of an external electric field, the liquid crystal molecules adopt a parallel orientation to the field axe and confer transparency to the PDLC film.
The electro-optical performances: transmittance, switching voltage (voltage necessary for passing from an opaque to a transparent state), the PDLC film response time . . . are limited amongst other thing, by the behavior of the liquid crystals at the polymer/liquid crystal interface. Indeed it has been noted, in certain prior art devices that the liquid crystals at the interface of the droplet orient differently from those at the center of the droplet, and are less sensitive to the application of an electric field. The droplet size also plays a role in the electro-optical properties of PDLC films. A reduced droplet size leads to an increase in anchoring and consequently in an increase in the switching voltage. On the other hand an increased droplet size is unfavorable to a minimum transmittance in the field-off state and deteriorates the contrast.
Preparation techniques of PDLC films are generally based either on phase separation methods or on emulsification methods.
Separation phase methods consist in allowing the liquid crystal to disperse in the polymeric matrix. There exists three phase separation methods: the PIPS method (polymerization induced phase separation), the TIPS method (thermally induced phase separation), and the SIPS method (solvent induced phase separation). These methods present disadvantages such as the possible presence of inhomogeneities in the polymer matrix, an irregular distribution of the liquid crystal droplets, or the instability of formed films. That is why these methods do not permit to obtain sufficient electro-optical performances for an optimal application in the field of intelligent glazing. Finally theses methods can be polluting due to the use of organic solvents and can be also costly due to the complexity of used processes.
Emulsification methods or encapsulation methods consist in using a water soluble polymer matrix. The NCAP (nematic curvilinear aligned phase) method for example, based on these encapsulation methods consists in dispersing an emulsion of liquid crystal droplets in a water soluble polymer solution. Droplets obtained by this method tend to be non-uniform in size and tend to coalesce. Park and al. for example, in U.S. Pat. Nos. 6,319,424 and 6,270,691, describe a liquid crystal emulsion stabilized by a copolymer. The copolymer is constituted of a hydrophilic monomer and of a hydrophobic monomer. The response when an external voltage is applied is not optimal in terms of transmittance.
Electro-optical properties of PDLC films greatly depend on the nature of the polymer matrix/liquid crystal interface, and improvements are still needed in this stage.
The object of the present invention is to overcome at least one of above mentioned disadvantages. The aim of the present invention is to design PDLC films with improved electro-optical performances, with an easy to implement preparation method which is non-polluting and cost effective. Another aim of the present invention is to produce a system that can be applied to large surfaces as well as non-planar surfaces, without sophisticated treatment.